In A/D converters (analog to digital converters), the input stage frequently is the most critical part of the converter in terms of noise and linearity. To increase linearity, degeneration of the input transistor pair is often used, but this at the same time increases noise. Another solution is to use an input stage which has feedback to input of the amplifier (like an inverting amplifier), but needs a resistive input, and therefore the input impedance of the A/D converter will be finite, which is not always desirable, in particular when the A/D converter needs to interface with a sensor. Furthermore, such feedback increases power consumption.
Furthermore, in sensor applications it is often required to have signal processing paths that provide adequate gain matching between the different channels. For instance, the output signals of a magnetic angular sensor are respectively proportional to the sine and cosine of the angle to be measured, the ratio of which can be processed by applying the arctangent function to give the angle of a magnetic field. Amplitude differences in the sine and cosine signals caused by mismatch in the independent signal processing paths give rise to angular errors. Therefore, gain matching between the signal processing paths is essential to achieve good performance.
FIG. 1 shows a schematic diagram of a sigma delta A/D converter 100, which achieves high resolution by oversampling noise shaping. More specifically, the converter 100 comprises an adder 104, a filter 106, a quantizer 108 and a feedback D/A converter 114. In operation, an analog input signal 102 is fed as one input to adder 104. The output of the adder 104 is fed to the filter 106, which shapes the quantization noise introduced by the following quantizer 108 running at high speed and providing digital output signal 110. The digital output signal 110 is also supplied as a digital feedback signal 112 to the feedback D/A converter 114 which forwards a resulting analog feedback signal 116 a second input of adder 104 in order to allow the filter 106 to take any error (i.e. difference between analog input signal 102 and analog feedback signal 116) into account. The filter 106 may be an integrator stage. The output of the converter 110 is converted into the analog domain by DA converter 114. The output of the DA converter 114 is compared with the input signal 102 and the error is fed back into the loop. Critical part of the converter 100 is the input stage, which determines the error of the digital representation of the analog input signal. An error made by this input stage itself therefore directly transforms into an error in the digital representation of the input signal which is unwanted. It is noted that SAR (successive approximation) AD converters have a similar issue. Prior art implementations use transconductance (gm) stages that independently convert the input signal and the DA converter signal.
FIG. 2 shows an example of a known input stage 200 for a differential A/D converter. The input stage 200 comprises a first transconductance element 206, a second transconductance element 210, an integrating capacitor 208 and a resistor ladder 216 (forming the feedback D/A converter) capable of providing an output signal value between +Vref and −Vref. The differential analog input signals are supplied to the inputs 202 and 204 of the first transconductance element 206 which generates a first current corresponding to the difference between the two input signals. Similarly, the second transconductance element 210 receives the analog feedback signals from the D/A converter 216 at its inputs 212 and 214 and generates a second current (opposing the first current) corresponding to the difference between the two analog feedback signals. The resulting current, i.e. the sum of the first current and the second current, is integrated on the capacitor 208 and the resulting voltage Vout across the capacitor 208 is provided for additional filtering (in case of a loop filter of higher order) or further processing in the quantizing stage of the converter, e.g. the quantizer 108 shown in FIG. 1. In other words, the output current of the first and second transconductance elements 206 and 210 are subtracted in the current domain, and integrated on the capacitor 208. Referring again to FIG. 1, the capacitor 208 may form the entire or a first part of the filter 108. A disadvantage of this implementation is that both transconductance elements have to convert the full swing signals (either the input signal or the DA converter output signal), which will require a highly linear stage to avoid distortion. Normally this linearity is obtained by degeneration of the transconductance stages, but obviously this increases noise (due to the added resistance), and therefore, power needs to be increased in order to achieve the desired noise floor.
There may thus be a need for an input stage for an A/D converter which is capable of providing high linearity and low noise at a lower power consumption, and which is capable of providing gain matching between multiple channels in a simple and reliable manner.